Wetting interaction between Sn-Zn-Ag solders and Cu

The wetting interaction of Sn-(7.1–9)Zn-(0–3)Ag solders with Cu was investigated from 230°C to 300°C. The wetting time, wetting forces, and activation energy of the wetting reaction were studied. The wetting time decreases with increasing temperature and increases with Ag content. The wetting force exhibits a disproportional correlation to temperature rise, while no trend was observed with respect to Ag content. The wetting behavior was ascribed to the interaction between Cu and Zn. The AgZn₃ compound was formed at the interface when the solder contains 0.3% Ag and above, while it was formed within the bulk solder at 2% Ag and above.     

Thermal diffusion of lithium acceptors into ZnO crystals

Electron paramagnetic resonance (EPR) has been used to monitor the diffusion of lithium ions into single crystals of ZnO. The in-diffusion occurs when a crystal is embedded in LiF powder and then held in air at temperatures near 750°C for periods of time ranging up to 22 h. These added lithium ions occupy zinc sites and become singly ionized acceptors (because the material is initially n type). A corresponding reduction in the number of neutral shallow donors is observed with EPR. To monitor the lithium acceptors, we temporarily convert them to the EPR-active neutral acceptor state by exposure to laser light (325 nm or 442 nm) at low temperatures. Also, after each diffusion treatment, we monitor the EPR signal of singly ionized copper acceptors and the photo-induced EPR signal of neutral nitrogen acceptors. These nitrogen and copper impurities are initially present in the crystal, at trace levels, and are made observable by the thermal anneals. Infrared-absorption measurements at room temperature in the 2–10 μm region show that the concentration of free carriers decreases as lithium is added to the crystal. After 22 h at 750°C in the LiF powder, the free-carrier absorption is no longer present, and the crystal is semi-insulating.     

Low-energy electron-enhanced etching of HgCdTe

Low-energy electron-enhanced etching (LE4) is applied to HgCdTe to eliminate ion-induced surface damage. First, LE4 results for patterned samples are illustrated. The LE4 mechanism is understood from a mechanistic study in terms of three etch variables: direct current (DC) bias, gas composition, and sample temperature. For this paper, the effects of DC bias (electron energy) and gas composition (CH₄ concentration) are summarized qualitatively, followed by quantitative evidence. Etch rate, the amount of polymer, surface stoichiometry, and surface roughness have specific relations with each etch variable under competition between pure LE4 and polymer deposition.     

Layer-transfer process for silicon-on-insulator with improved manufacturability

We observe hydrogen platelet buildup in single-crystalline silicon caused by hydrogen-plasma processing. The platelets are aligned along a layer of lattice defects formed in silicon before the plasma processing. The buried-defect layer is formed by either silicon-into-silicon or argon-into-silicon implantation. We discuss the platelet nucleation, growth, and merge phenomena and discuss applicability of the plasma hydrogenation to silicon-on-insulator (SOI) wafer fabrication by layer transfer.     

Effect of silver content on thermal fatigue life of Sn-xAg-0.5Cu flip-chip interconnects

The thermal fatigue properties of Sn-xAg-0.5Cu (x=1, 2, 3, and 4 in mass%) flip-chip interconnects were investigated to study the effect of silver content on thermal fatigue endurance. The solder joints with lower silver context (x=1 and 2) had a greater failure rate compared to those with higher silver content (x=3 and 4) in thermal fatigue testing. Cracks developed in the solders near the solder/chip interface for all joints tested. This crack propagation may be mainly governed by the nature of the solders themselves because the strain-concentrated area was similar for tested alloys independent of the silver content. From the microstructural observation, the fracture was a mixed mode, transgranular and intergranular, independent of the silver content. Higher silver content alloys (x=3 and 4) had finer Sn grains before thermal cycling according to the dispersion of the Ag₃Sn intermetallic compound, and even after the cycling, they suppressed microstructural coarsening, which degrades the fatigue resistance. The fatigue endurance of the solder joints was strongly correlated to the silver content, and solder joints with higher silver content had better fatigue resistance.     

Microstructure characterization of Sn-Ag solder joints between stud bumps and metal pads

The microstructure of the flip-chip solder joints fabricated using stud bumps and Pb-free solder was characterized. The Au or Cu stud bumps formed on Al pads on Si die were aligned to corresponding metal pads in the substrate, which was printed with Sn-3.5Ag paste. Joints were fabricated by reflowing the solder paste. In the solder joints fabricated using Au stud bumps, Au-Sn intermetallics spread over the whole joints, and the solder remained randomly island-shaped. The δ-AuSn, ε-AuSn₂, and η-AuSn₄ intermetallic compounds formed sequentially from the Au stud bump. The microstructure of the solder joints did not change significantly even after multiple reflows. The AuSn₄ was the main phase after reflow because of the fast dissolution of Au. In the solder joints fabricated using Cu stud bumps, the scallop-type Cu₆Sn₅ intermetallic was formed only at the Cu interface, and the solder was the main phase. The difference in the microstructure of the solder joints with Au and Cu stud bumps resulted from the dissolution-rate difference of Au and Cu into the solder.     

Soft quasi-maximum-likelihood detection for multiple-antenna wireless channels

The paper addresses soft maximum-likelihood (ML) detection for multiple-antenna wireless communication channels. We propose a soft quasi-ML detector that maximizes the log-likelihood function by deploying a semi-definite relaxation (SDR). Given perfect channel state information at the receiver, the quasi-ML SDR detector closely approximates the performance of the optimal ML detector in both coded and uncoded multiple-input, multiple-output (MIMO) channels with quadrature phase-shift keying (QPSK) modulation and frequency-flat Rayleigh fading. The complexity of the quasi-ML SDR detector is much less than that of the optimal ML detector, thus offering more favorable performance/complexity characteristics. In contrast to the existing sphere decoder, the new quasi-ML detector enjoys guaranteed polynomial worst-case complexity. The two detectors exhibit quite comparable performance in a variety of ergodic QPSK MIMO channels, but the complexity of the quasi-ML detector scales better with increasing number of transmit and receive antennas, especially in the region of low signal-to-noise ratio (SNR).     

Secure encrypted-data aggregation for wireless sensor networks

This paper proposes a secure encrypted-data aggregation scheme for wireless sensor networks. Our design for data aggregation eliminates redundant sensor readings without using encryption and maintains data secrecy and privacy during transmission. Conventional aggregation functions operate when readings are received in plaintext. If readings are encrypted, aggregation requires decryption creating extra overhead and key management issues. In contrast to conventional schemes, our proposed scheme provides security and privacy, and duplicate instances of original readings will be aggregated into a single packet. Our scheme is resilient to known-plaintext attacks, chosen-plaintext attacks, ciphertext-only attacks and man-in-the-middle attacks. Our experiments show that our proposed aggregation method significantly reduces communication overhead and can be practically implemented in on-the-shelf sensor platforms.     

Curvelet-based fingerprint anti-spoofing

Existing perspiration-based liveness detection algorithms need two successive images (captured in certain time interval), hence they are slow and not useful for real-time applications. Liveness detection methods using extra hardware increase the cost of the system. To alleviate these problems, we propose new curvelet-based method which needs only one fingerprint to detect liveness. Wavelets are very effective in representing objects with isolated point singularities, but fail to represent line and curve singularities. Curvelet transform allows representing singularities along curves in a more efficient way than the wavelets. Ridges oriented in different directions in a fingerprint image are curved; hence curvelets are very significant to characterize fingerprint texture. Textural measures based on curvelet energy and co-occurrence signatures are used to characterize fingerprint image. Dimensionalities of feature sets are reduced by running Pudil’s sequential forward floating selection (SFFS) algorithm. Curvelet energy and co-occurrence signatures are independently tested on three different classifiers: AdaBoost.M1, support vector machine and alternating decision tree. Finally, all the aforementioned classifiers are fused using the “Majority Voting Rule” to form an ensemble classifier. A fingerprint database consisting of 185 real, 90 Fun-Doh and 150 Gummy fingerprints is created by using varieties of artificial materials for casts and moulds of spoof fingerprints. Performance of the new liveness detection approach is found very promising, as it needs only one fingerprint and no extra hardware to detect vitality.     

Co Solubility in Sn and Interfacial Reactions in Sn-Co/Ni Couples

It has been reported that minute Co additions to Sn-based solders are very effective for reducing undercooling, probably due to low Co solubility in Sn. In this study, Co solubility in molten Sn was determined experimentally. According to results of metallographic analysis, Co solubility in molten Sn is as low as 0.04 wt.% at 250°C. Interfacial reactions in Sn-Co/Ni couples at 250°C were examined for Co contents from 0.01 wt.% to 0.4 wt.%. The Ni₃Sn₄ phase was the only interfacial reaction phase in almost the entire Sn-0.01 wt.%Co/Ni couple. For Sn-Co/Ni couples with a Co content higher than 0.01 wt.%, a thin, continuous Ni₃Sn₄ layer and a discontinuous decahedron (Ni,Co)Sn₄ phase were formed in the initial stage of reaction. The reaction products evolved with time. With longer reaction time, the Sn content in the decahedron (Ni,Co)Sn₄ phase decreased, and the (Ni,Co)Sn₄ phase transformed into the (Ni,Co)Sn₂ phase and cleaved into a sheet, which then detached from the interface, after which Ni₃Sn₄ began to grow significantly with longer reaction times.